1. Field of the Invention
The present invention relates generally to a wavelength division multiplexing device having a function of compensating for the dispersion that occurs while optical pulses propagate along optical fiber, and, more particularly, to a wavelength division multiplexing device including a fiber Bragg grating for compensating for dispersion.
2. Description of the Related Art
In the present invention described below, the term “wavelength division multiplexing device” is used as a concept, including an Add-Drop Multiplexer (ADM) for selectively adding/dropping specific wavelength channels to/from input wavelength channels and an interleaver for dividing input wavelength channels in two and outputting the resulting wavelength channels.
An ADM is a core component of an optical communication system that is used to perform Wavelength Division Multiplexing (WDM). Various methods may be used to construct an ADM, but, recently, a method of constructing an ADM by interposing a Fiber Bragg Grating (FBG) between two optical circulators has been proposed.
In an ADM using the above-described FBG, the FBG can be produced by varying the refractive index in optical fiber at regular intervals. Methods of varying a refractive index include amplitude sampling and phase sampling. For reference, the variation of the refractive index of an amplitude-sampled FBG within an optical fiber core is shown in FIG. 1. In FIG. 1, the x-axis represents the length of the optical fiber and the y-axis represents the refractive index of the optical fiber. It can be understood from the FBG of FIG. 1 that grating spacings are the same, but the amplitude of the refractive index varies. A grating producing method shown in FIG. 1 is called “amplitude sampling” due to the above-described phenomenon. In contrast, a phase-sampled FBG is related to the case where grating spacings are the same and amplitude is constant.
Methods of manufacturing the above-described amplitude-sampled FBG and phase-sampled FBG are well known in the technical field to which the present invention pertains. Of the manufacturing methods, a phase masking method is widely used. FIG. 2 shows a grating manufacturing method based on the phase mask method. As shown in FIG. 2, when a laser beam 11 generated by a laser source 10 is irradiated onto optical fiber 40 through a phase mask 30 in which a specific pattern is formed, the refractive index of the regions of the optical fiber 40, onto which the laser beam is irradiated in conformity with the pattern, varies. In this method, a mirror 20 for reflecting the laser beam 11 is moved to allow the laser beam 11 to irradiate specific portions of the optical fiber along the phase mask 30. At this time, when the moving speed of the mirror 20 varies, an amplitude-sampled grating can be produced. When the moving speed of the mirror 20 is made constant and the phase mask is appropriately laterally moved, a phase-sampled grating can be produced. In this case, the production of a grating in optical fiber is called index modulation.
Meanwhile, the above-described amplitude-sampled FBG is problematic in that index modulation must be increased in proportion to the number of channels. Since index modulation is obtained by the irradiation of a laser beam as described above, a larger amount of index modulation can be obtained through the longer irradiation of a laser beam. However, there is a limitation in varying a refraction index in optical fiber, so that the amplitude sampling has a limitation in that it cannot be applied to a wavelength division multiplexing device having a large number of channels.
In the meantime, the transmission of optical signals through optical fiber is problematic in that the dispersion of an optical signal occurs. For normal fiber at 1550 nm, a short wavelength optical fiber has a short delay time, while a long wavelength optical fiber has a long delay time. Delay time has characteristics of varying non-linearly depending on wavelength. Accordingly, to fully compensate for such non-linear dispersion characteristics, compensation for dispersion and a dispersion slope is required.
Loh et at. proposed a dispersion slope compensation method using amplitude-sampled FBGs in the thesis entitled “Sample fiber grating based-dispersion slope compensation” (IEEE photon., Technol. Lett., Vol. 11, No. 10, pp. 1280–1282).
However, the above-described document employs an interleaving technique with an increase in the number of wavelength channels by alternating amplitude-sampled FBGs to efficiently utilize the length of optical fiber. In this method, FBGs having a shape similar to that of phase-sampled FBGs are used by completely eliminating the intervals between the amplitude-sampled FBGs, or a plurality of amplitude-sampled FBGs, which functions independently because the Bragg wavelengths of the amplitude-sampled FBGs are sufficiently spaced apart from each other, are used.